Relieving Mipafox Inhibition in Organophosphorus Acid Anhydrolase by Rational Design

Organophosphate acid anhydrolase OPAA is a bimetalloenzyme that hydrolyzes acetylcholinesterase-inhibiting organophosphorus compounds, including fluorine-containing nerve agents such as soman pinacolyl methylphosphonoflouridate. The insecticide mipafox N,N-diisopropyldiamidofluorophosphate, which is a close analog of the nerve agent substrate diisopropyl fluorophosphonate, is a special case for OPAA because it reversibly inhibits the enzymes hydrolysis activity. It is believed that OPAA hydrolyzes mipafox to DDP N,N-diisopropyldiamidophosphate, which becomes bound to the OPAA active site through a hydrogen bond network. Evaluation of the crystal structure of the OPAA-DDP complex suggests that disruption of two of these bonds, one involving Asp244 and the other involving Glu381, could potentially release DDP from the OPAA active site to give the enzyme catalytic mipafox hydrolysis activity. To test this hypothesis, we produced three mutant OPAA enzymes including one enzyme with an Asp244-to-alanine mutation, a second with a Glu381-to-alanine mutation, and a third with both mutations, to remove the corresponding hydrogen bonds between these amino acids and the bound DDP. In all cases, the mutant enzymes did not have mipafox hydrolysis activity but retained their high hydrolysis rates against soman, which indicated that the remaining OPAA-DDP interactions were adequate to stabilize their complex and keep DPP bound to the enzyme.